The present invention relates to material processing using a laser beam and, more particularly, to an improved method and apparatus for material processing, such as drilling, cutting, machining or the like, using an improved multi-pass laser trepanning method.
The use of lasers for material processing are finding wider applications in industry. The CO.sub.2 laser and the yttrium-aluminum-garnet (YAG) face-pumped laser (FPL) are particularly suitable for material processing of super alloys and other exotic materials currently being used in the manufacturing of high performance gas turbine engines for use in aircraft propulsion as well as other applications.
A CO.sub.2 laser and YAG-type FPL each have linearly polarized beams. While the CO.sub.2 laser has a substantially circular beam, the YAG-type FPL 10 has a substantially rectangularly shaped beam 12 or footprint, as shown in FIG. 1, which is linearly polarized in a direction substantially perpendicular to the longest sides of the rectangularly shaped beam 12, as indicated by oppositely pointing arrows 14. The rectangularly shaped laser beam 12 in FIG. 1 is shown to be much larger than the actual beam size for purposes of illustration. In operation, as laser beam 12 or workpiece 16 are moved relative to one another in a counterclockwise direction, illustrated by arrow 18, in a circular path illustrated by broken line 20, the energy of laser beam 12 couples better with the host material of workpiece 16 in the direction of polarization 14. Because the laser energy couples better with the material in the direction of polarization, more energy is absorbed by the host material in the polarization direction. As laser beam 12 moves along cutting or trepanning path 20, the leading faces or sides of rectangular laser beam 12 will couple more into the material and the actual path cut by laser beam 12 will be substantially elliptical as illustrated by chain line 22. The longer and shorter sides of rectangular beam 12 each have distinctly different cutting cross sections; however, whatever the beam orientation, the actual trepanning or cutting path may differ from the path followed by the focal point of the laser beam and the difference between these paths may be more or less severe according to the beam orientation.
For the example shown in FIG. 21, as laser beam 12 is moved arcuately upward along cutting path 20 toward the Y axis in quadrant I, the left longer side 24 of laser beam 12 will actually cut inside of desired cutting path 20 and will actually cut along path 22. Thus, laser face 24 couples or cuts more into the host material in the I and II quadrants and the right longer side 26 of laser beam 12 will couple or cut more into the host material in the III and IV quadrants as shown in phantom 12' in FIG. 1.
Similarly, referring to FIG. 1B if the focal point of laser beam 12 is moved in a clockwise direction 28 along circular cutting path 20, the actual path cut or trepanned will be substantially elliptical as shown by chain line 22' except that the major axis of the ellipse will lie in the I and III quadrants and the minor axis will lie in the II and IV quadrants which is opposite to those shown in FIG. 1A. The difference between the desired cutting path 10 or path of the beam focal point and the actual or resulting cutting paths 22 and 22' are exaggerated in FIGS. 1A and 1B for purposes of illustration; however, the ellipticity or difference between the path of the laser beam focal point and the actual path cut may be undesirable, particularly in the manufacture of gas turbine engines and other aerospace vehicle engines where precise tolerances are required.
While the examples shown in FIGS. 1A and 1B were explained using a YAG-type FPL, because the CO.sub.2 laser also has a linearly polarized beam, it may also result in an actual cutting or trepanning path which is different from the path followed by the focal point of the laser beam during a drilling or machining operation. The rectangular shape of the YAG-type FPL may, however, compound the difference between these paths because each face of the rectangular beam has a distinctly different cutting cross section. Additionally, while the asymmetrical cutting characteristics of a linearly polarized laser beam where described in FIGS. 1A and 1B with respect to drilling a substantially circular hole, those skilled in the art will recognize that a linearly polarized laser beam can also cause inaccuracies in drilling or machining precision holes or features in a workpiece surface which have a shape other than circular.
The adverse effects of a linearly polarized laser beam can be diminished to some extent by passing the linearly polarized laser beam 12 through a quarter wave plate or lens (not shown in FIGS. 1A and 1B) to convert the linearly polarized laser beam 12 to a beam having a polarization of a random format. The lens, however, will cause some reflection and diffraction of the laser beam which will cause loss of some energy for performing the cutting or trepanning operation. The method of the present invention eliminates the need for a quarter wave plate or any other intermediate optical devices between the laser system and the workpiece, and the present invention actually takes advantage of the asymmetrical cutting characteristics of the linearly polarized FPL or CO.sub.2 laser as described hereinabove to provide higher material processing rates.
A prior art method and device for producing bores or holes of substantially circular cross section in workpieces, particularly in watch jewels, by means of a laser beam, is disclosed in U.S. Pat. No. 3,576,965, issued to Gugger. Gugger, however, discloses a glass disc mounted in an inclined position of engagement within a tube. During operation of the device, the tube and disc are rotated about a central axis which is congruent with the direction of propagation of a laser beam within the tube. The inclined disc causes the laser beam propagating through the tube to be diffracted from the central axis and as the tube rotates about the central axis the diffracted laser beam will also rotate along a circular path around the central axis. The glass disc adds distortion to the laser beam and will cause some reflection of the beam resulting in loss of laser energy for performing cutting at the workpiece. Additionally, Gugger does not teach or suggest how to compensate for the asymmetrical cutting characteristics of a linearly polarized laser such as a FPL or CO.sub.2 type laser, if such a laser were to be used with the device of Gugger. The device and method disclosed by Gugger may even exaggerate the asymmetrical cutting characteristics if used with a linearly polarized laser. Gugger is also limited with respect to the depth and diameter of the hole or bore which can be produced because the workpiece and device basically remain stationary relative to one another and only the tube and glass disc are rotated in a fixed location to cause diffraction of the laser beam and rotation of the beam about the central axis. Thus, the diameter of the hole drilled by Gugger will be limited by the size of the glass disc and the degree of inclination of the disc. Additionally, the hole depth will be limited because of the divergence of the laser beam as the hole gets deeper.
Another method and apparatus for trepanning a workpiece using laser energy is disclosed in U.S. Pat. No. 4,896,944, issued to Irwin et al. Irwin discloses directing a collimated beam of laser energy onto a workpiece and shifting and rotating a focusing lens to cause the collimated laser beam to orbit a focal point on the workpiece to change the trepanning diameter of the hole being bored. Irwin also does not teach or suggest how to correct for the asymmetrical cutting characteristics caused by a linearly polarized laser beam, if a linearly polarized laser beam is used with Irwin. As with Gugger, the device and method of Irwin may even exaggerate the asymmetrical cutting characteristics if the Irwin apparatus and method are used with a FPL or a CO.sub.2 laser which both generate a linearly polarized laser beam. Additionally, the focusing lens of Irwin causes the laser beam to diffract and will add distortion to the beam which causes loss of laser energy incident upon the workpiece for performing cutting and thereby reduces the efficiency of the laser. Furthermore, the apparatus and method of Irwin are also limited with respect to the diameter and depth of a hole which can be produced because the apparatus and workpiece basically remain stationary and the laser beam is rotated by rotation of the focusing lens. Therefore, the hole diameter is limited by the size of the focusing lens and the extent to which the focusing lens is permitted to travel to its offset positions within the nozzle housing of the laser apparatus.
A further device for trepanning a hole in a workpiece using laser energy is disclosed in Japanese Patent No. 1-228692(A). The English abstract of the Japanese patent discloses that the laser beam is reflected by first, second and third rotating reflection plates to bore a hole of a selected diameter through a workpiece without rotating the workpiece. The English abstract of the Japanese patent does not teach or suggest correction of the asymmetrical cutting characteristics if a linearly polarized laser beam is used with the apparatus disclosed. Additionally, the three rotating reflection plates will introduce losses which will reduce the amount of laser energy incident upon the workpiece for performing the boring operation and therefore reduce the cutting speed and depth of the cut taken by each pulse of the laser. The diameter of the hole bored is also limited by the size of the reflection plates.
In summary, there prior art devices and methods have limitations, particularly if used with a linearly polarized laser beam, and appear to only provide for movement of the laser beam in a single rotational direction and are limited to circular cutting paths.
In a standard laser trepanning operation to cut a large diameter hole, much larger than the focal point of the laser beam, through a piece of material having a thickness greater than about 0.5", the laser beam is initially focused at one location on the workpiece and the laser beam is continuously pulsed to drill a single hole, about the size of the laser beam footprint, completely through the workpiece by percussion. After drilling completely through the workpiece then either the laser apparatus or the workpiece are moved relative to one another so as to bore out another portion of material immediately adjacent and adjoining the hole previously bored. In this manner, either the laser apparatus or the workpiece are stepped relative to one another, in the same direction, as successive portions of material are drilled out around a plug of material which will fall out after the last portion of material is removed to form a large diameter hole through the workpiece. This method for forming a large diameter hole is referred to as a single pass cutting method and is not limited to circular shaped holes. Because either the laser apparatus or the workpiece can be moved relative to one another, allowances for the nonsymmetrical cutting characteristics of an FPL or a CO.sub.2 laser can be made; however, because the laser beam remains stationary at each location as each successive portion of material is drilled out by the laser beam, a heat-affected zone may be created around the location where the laser beam is incident upon the workpiece. High heat can be produced in the heat-affected zone which can thermally damage the host material by changing the grain structure or mechanical properties of the material and an undesirable increase in recast material may form around the boundary of the large dimension hole which may require subsequent processing steps to remove. This laser trepanning method also has a limited processing rate because the relative travel speed of the laser or component along the prescribed cutting or trepanning path is restrained to a speed which will allow the laser to maintain complete breakthrough of the component material. The travel speed may be as low as about 0.05 inches per minute (IPM) for a superalloy material, such as Rene' 88, a nickel-aluminide or the like, having a thickness of about 1".